1. Field of the Invention
The present invention relates to the field of the technology of coding digital image data such as a frame image. In particular, the present invention relates to the field of the image coding technology of carrying out motion prediction in an identical frame (intra prediction).
2. Description of the Related Art
As related technologies, the following patent documents are found for example.
That is, JP-Laid Open Patent No. 232886/2002 (Paragraphs 48 and 49, and FIG. 5) proposes the technology of referring to the coding modes of already coded blocks existing in the vicinity of a block to be coded based on the statistical properties thereof when the block to be coded is coded in an image encoder.
Further, JP-Laid Open Patent No. 186823/1996 (Paragraphs 37 and 43, and FIG. 17) proposes the technology of, when image data is coded: selecting an optimum difference value generating method (a coding mode) in each block of 4×4 pixels to be coded; generating a difference value by the selected difference value generating method; and then coding the generated difference value.
Moving picture compression technologies such as MPEG (Moving Picture Experts Group) 2, MPEG 4 and others have heretofore been adopted in various kinds of image processors that handle moving pictures.
In the moving picture compression technologies (moving picture coding technologies, so-called codec) such as MPEG 2, MPEG 4 and others, each pixel that composes a frame image (hereunder referred to simply as “frame” occasionally) is not actually recorded. In such moving picture compression technologies, in order to reduce the amount of moving picture data, the image compression is carried out based on the result obtained by predicting where an image block contained in a certain frame moves in other subsequent frames (namely, with the lapse of time).
As a technology of expanding such conventional moving picture compression technologies, H.264/AVC (Advanced Video Codec) has been a focus of constant attention in recent years. H.264/AVC is a technology cooperatively developed by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) and ISO (International Organization for Standardization)/IEC as a moving picture compression technology which is intended to be an open standard. H.264/AVC codec is also called MPEG 4 AVC.
Even in such H.264/AVC, the basic concept of the image compression technology thereof is in common with that of the aforementioned conventional moving picture compression technologies such as MPEG2 or MPEG4. A difference is that the function called “sub-macro block splitting” is adopted in H.264/AVC. Thereby, compression processing which is more flexible than the conventional moving picture compression technologies can be realized with a less amount of data. Here, a sub-macro block means a block of a smaller splitting unit (for example, a block of 4×4 pixels) that composes a macro block (for example, a block of 16×16 pixels).
Further, in H.264/AVC, by adopting the function called “intra prediction,” motion prediction in an identical frame that has not been realized in the aforementioned conventional moving picture compression technologies is realized.
In the meantime, there are two main types of methods for the intraframe prediction of H.264. One is a method of making prediction for each macro block of 16×16 pixels (hereunder referred to simply as “16×16 macro block”) and the other is a method of making prediction for each sub-macro block of 4×4 pixels (hereunder referred to simply as “4×4 block”). Then, in the method of making motion prediction for each 4×4 block, nine modes are defined as the candidates of the prediction result. On the other hand, in the method of making motion prediction for each macro block of 16×16 pixels, four modes are defined as the candidates of the prediction result. Therefore, though it also depends on how the codec is implemented, it is considered that, generally speaking, the motion prediction for each 4×4 block allows a closer prediction but tends to increase the amount of calculation in comparison with the motion prediction for each 16×16 macro block.